Measurement setups with conductivity measuring cells, which work according to the conductive measurement principle, wherein an alternating voltage is placed on electrodes directly in contact with a liquid medium and the electrical signal response proportional to the conductivity of the liquid medium is evaluated in a measuring device, are known from analytical and process measurements technology. German Patent DE 198 44 489 A1 discloses such a method for determining the electrical conductivity of liquids. The evaluation of the electrical signal response occurs, in such case, based on an equivalent circuit of the conductivity measuring cell, which is formed at least of a capacitor and a resistor connected in parallel therewith. In order especially to prevent disturbing polarization effects, the area of the electrodes and their separation from one another are matched on the measuring range of the conductivity measuring cell. This matching delivers a cell constant typical for each conductivity measuring cell. The cell constant is subject to production-related fluctuations and can change as a function of time, so that a measurement setup with such a conductivity measuring cell must, in given cases, be calibrated.
For this, the conductivity measuring cell is supplied with calibration liquids of predetermined conductivity and the resistance measured, for example, using an alternating voltage of 1 kHz. From the resistance, by means of the equivalent circuit, the cell constant is determined. For preventing measurement errors, the conductivity of the calibration liquids should lie in the range of the conductivities of the liquid medium to be measured. Especially in the case of liquid media with smaller conductivity, for example, very pure water, the providing of stable calibration liquids requires considerable effort. The application of reference cells, which precisely determine the conductivity of the liquid medium and thereby permit a calibrating of the cell constant by means of the so won conductivity of the liquid medium, just shifts the calibration effort by means of calibration liquids to the reference cell. Furthermore, in the case of installation in a measuring container, such as a vessel, additional container openings must be provided. The calibration is furthermore made difficult by the large temperature dependence of the conductivity measuring cell during a calibration by means of calibration liquids, which supplementally requires complex temperature measurement during the calibration with corresponding settling times of temperature especially between the conductivity measuring cell to be calibrated and the reference cell.